Organic materials with semiconducting or charge-transport properties have recently shown promise as the active layer in organic based thin film transistors (TFT) and organic field effect transistors (OFETs) [see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
Furthermore, these organic materials have been proposed for use in light-emitting and electroluminescent (EL) devices like organic light-emitting diodes (OLED).
The performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2V−1s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
In prior art polythiophene (PT), for example regioregular head-to-tail (HT) poly-(3-alkylthiophene) (PAT), in particular poly-(3-hexylthiophene) (PHT), has been suggested for use as semiconducting material, as it shows charge carrier mobility between 1×10−5 and 0.1 cm2V−1s−1. Also, PAT shows good solubility in organic solvents and is solution processable to fabricate large area films.
In order to fabricate TFT, OFET and OLED devices usually low cost, solution based, additive processes are used. For example, ink-jet printing of solutions of electroluminescent or semiconducting polymers has been suggested in prior art as a suitable method.
For this purpose it is necessary to prepare stable inks or formulations of the polymeric semiconductor material. These polymers, like for example the commonly used PAT, however, do generally have solubility problems for two main reasons.
Firstly, semiconducting polymers require a conjugated component, generally the backbone, to confer the appropriate electronic bandgap. This backbone usually contains a high degree of aromatic hydrocarbon and heterocyclic rings, which tend to limit the solubility in coating solvents. Introduction of solubilising groups, like for example the alkyl-groups in case of PAT, has the effect of both diluting the electronically active fraction of the material, thus lowering the electrical performance, and may additionally trap charge, depending on the functional groups used. These groups are generally therefore only incorporated sparingly into the polymer structure.
Secondly, charge transport polymers are molecularly designed to facilitate intermolecular charge hopping, which is enhanced by close packing and aggregation of the polymer chains. When this happens in solution, gelling occurs, which, if irreversible, leads to filtration and coating problems.
Therefore, there is a need to provide suitable compositions or formulations comprising semiconducting polymers like PAT which are suitable for the preparation of electronic and EL devices like TFTs, OFETs and OLEDs, in particular by ink-jet printing.
Compositions or formulations comprising semiconducting polymers and organic solvents have been described in prior art.
U.S. Pat. Nos. 5,069,823 and 4,737,557 disclose PAT and methods of its preparation and also mention solutions of PAT in tetralin.
US 2003/116772 A1 discloses the fabrication of a light-emitting device from an organic light-emitting compound dissolved in a solvent by coating methods. As possible organic compound the reference generally discloses polymers such as PT, PPV, PP or PF are disclosed. As possible solvents the reference generally mentions toluene, benzene, (di)chlorobenzene, chloroform, tetralin, xylene, DCM, cyclohexane, NMP, DMSO, cyclohexanone, dioxane, THF and the like.
U.S. Pat. No. 5,814,376 discloses a gravure coating process for forming a film of an electroconductive polymer on a substrate. As possible polymer polyacetylene, polypyrrole, PT, PAT, polyphenylene sulfide, PPV, polythienylene vinylene, polyphenylene (PP), polyisothianaphthene, polyazulene, polyfuran or polyaniline are generally mentioned. As possible solvents chloroform, toluene, xylene, THF, DCM and tetralin are generally mentioned. Example 6 further discloses a mixture of PHT in a solvent mixture of toluene/xylene/indane 85/10/5.
However, the above documents do not mention the preparation of semiconducting or EL devices by ink-jet printing. Nor do they provide any technical teaching how to select the solvents and polymers in order to improve the print performance.
EP 0 880 303 A1 discloses a method of producing an organic EL device by ink-jet printing of a liquid composition including an organic emitting polymer like for example PAT, polyphenylenevinylene (PPV) or polyfluorene (PF). The reference further mentions some desired properties of the compositions like viscosity and surface tension, however, the disclosed ranges are very broad and do not provide a technical teaching how to select suitable solvents. There is no disclosure or suggestion for specific suitable solvents that can be used in the compositions.
WO 99/39373 A2 discloses a method of fabricating semiconductor devices by depositing a semiconducting material in a solvent onto a substrate by ink-jet printing. However, the reference does specifically disclose only a solution of polyvinylcarbazole (PVK) and light-emitting dyes in chloroform.
WO 01/47045 A1 discloses a method of forming an electronic device on a substrate from electrically conductive or semiconductive materials mixed with a liquid, wherein the mixture may, among other methods, also be deposited on the substrate by ink-jet printing. As semiconductive materials PHT, PF or copolymers of fluorene and thiophene are disclosed, and as solvents mixed xylenes.
US 2003/008429 A1 discloses a method of fabrication of a light-emitting device using ink-jet printing of a composition comprising an organic light-emitting compound and a solvent. As possible organic compound polymers like PPV, PT, (PF), polyparaphenylene (PPP), polyalkylphenylene or polyacetylene are generally mentioned. As possible solvents toluene, benzene, (di)chlorobenzene, chloroform, tetralin, xylene, anisole, dichloromethane (DCM), γ-butyrolactone, cyclohexane, N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), cyclohexanone, dioxane, tetrahydrofuran (THF) or the like are generally mentioned. However, there is no technical teaching how to select the solvents and polymers in order to improve the print performance. Embodiment 3 describes a light emitting layer formed by ink-jet printing of a solution of PVK and a light-emitting metal chelate complex without specifying the solvent.
WO 02/72714 A1 discloses solutions and dispersions comprising an organic semiconductor and a mixture of at least two solvents, wherein these solvents have a boiling point of less than 200° C. and a melting point of 15° C. or less, do not comprise benzylic CH2— or CH— groups, are no benzene derivatives with tert butyl or more than two methyl substituents, and at least one of these solvents has a boiling point of between 140° C. and 200° C. As semiconductors substituted polyarylenevinylenes (PAV), PFs, polyspirobifluorenes (PSF), polyparaphenylenes (PPP), PTs, polypyridines (PPy), polypyrroles, PVKs and polytriarylamines are mentioned. As preferred solvents for example xylol, anisol, toluol and their fluorinated, chlorinated or methylated derivatives are mentioned, whereas tetralin is reported as being unsuitable.
EP 1 083 775 A1 discloses a composition for use in the preparation of organic EL devices by ink-jet printing. The composition comprises an organic EL material and a first solvent comprising at least one substituted benzene derivative having a substituent with at least 3 carbon atoms, and preferably having a boiling point of at least 200° C., or a mixture of said first solvent with a second solvent having a boiling point of 140° C. or higher. As possible EL material polymers like PPV, PP, PT, PF or PVK are generally mentioned. As solvents, among others, cumene, cymene, cyclohexylbenzene, several alkylated benzenes, tetralin or mixtures thereof are proposed. Especially preferred are dodecylbenzene and 1,2,3,4-tetramethylbenzene. Specifically mentioned are furthermore mixtures of tetralin with xylene, dodecylbenzene or 1,2,3,4-tetramethylbenzene.
However, the formulations described in prior art have several drawbacks. For example, in case of solutions of polythiophene in chlorinated solvents like chloroform or trichlorobenzene, over time the solvents have been demonstrated to degrade the polythiophene backbone, thus lowering its electrical performance. In addition, chloroform is too volatile and prematurely evaporates during processing leading to coating irregularities, dispensing problems, loss of resolution and feature definition, as well as clogging of, for example, ink-jet heads. The solubility of polymers like for example polythiophene in toluene and xylene, on the other hand, is often not high enough to provide sufficient ink viscosity, nor can these solvents achieve a high enough surface tension for reliable ink-jet printing.
Therefore, there is a need for improved solutions and formulations of semiconducting polymers, in particular polythiophenes like 3-alkyl polythiophene (PAT), which do not have the drawbacks of prior art materials as described above, and which are suitable for processing the polymer in particular when manufacturing electronic or electrooptic devices like thin film organic transistors.
It was an aim of the present invention to provide improved formulations and inks, in particular solutions, that do not have the drawbacks of prior art materials mentioned above and are suitable for preparation of electronic, electrooptic and light-emitting devices in particular by ink-jet printing, in an economical, effective and environmentally beneficial way which is especially suitable for industrial large scale production.
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors of the present invention have found that these aims can be achieved and the above problems can be solved by providing formulations of semiconducting polymers as described below.